THE ACOUSTIC COMPLEX OF THE OPOSSUM 429 



Passing now to the horizontal sections, we find them rather 

 better adapted than the transverse to the picturing of the general 

 relations and structure of the vestibular system in the medulla. 



Fig. 10. Series B, slide 75, section 1. This section is taken 

 at the level of the broadest part of the combined medial and gpinal 

 nuclei of the vestibular nerve, and presents the arrangement of 

 the structures in the right wall and floor of the fourth ventricle. 

 The picture of the medial vestibular is one of the best in the 

 series. In the general topography the brachium conjunctivum, 

 the restiform body and the pons are conspicuous landmarks. 

 The two cochlear nuclei appear in typical relation to each other 

 and to the corpus restiforme, the anterior part of the ventral 

 nucleus being also clearly shown in its association with the region 

 of Deiter's and the nucleo-cerebellar tract. The corpus ponto- 

 bulbare on the lateral aspect of the corpus restiforme is better 

 shown in the following section. In drawing the medial and spinal 

 vestibular nuclei an attempt was made to convey the effect of 

 uniformity in staining reaction, which is so striking a feature of 

 this mass, even in the best differentiated sections. The absence 

 of any visible division into nucleus intercalatus and medial 

 vestibular nucleus is apparent, the peculiar brownish background 

 and the felt-work of exceedingly delicate fibers and small ganglion 

 cells being uniform throughout. There is however, a marginal 

 concentration of fibers along the inner border of the medial 

 nucleus. Some of these pass posteriorly as a slender tail-like ex- 

 tension towards the anterior end of the hypoglossal nucleus. The 

 larger part, however, appear better in the following figure, as the 

 anterior tongue or extension already mentioned as lying on the 

 medial side of the olivo-cochlear tract. The caudal indentation 

 in the medial nucleus due to the nucleus N. vagi and the tractus 

 solitarius, and the position of the fifth on the postero-medial 

 border, are well shown. Of Deiter's only the lateral portion can 

 be seen, among the strands of the nucleo-cerebellar tract. The 

 typical inward curve of those strands of the spinal tract which pass 

 into the nucleus superior N. vestibularis (the tract itself lies ven- 

 tral to this section) gives an index to the anterior limits of the 



